1. Field of the Invention
The present invention relates to a method for controlling ignition parameters of a spark plug for an internal-combustion engine and to an ignition device using such a method.
2. Description of the Prior Art
It is already well-known to use ignition coils to generate a high voltage applied to a spark plug in an engine. The high voltage applied to the spark plug creates, at the electrodes thereof, an electric arc whose energy is used to ignite an air/fuel mixture present in the combustion chamber of the engine.
These ignition coils generally consist of a primary winding, referred to as a primary, connected to a source of direct current, such as the battery of a motor vehicle, and a secondary winding, referred to as a secondary, connected to a spark plug and which, under the effect of the voltage applied to the primary, generates a high voltage which is applied to the spark plug so as to form, at the electrodes thereof, an electric arc or spark that ignites the air/fuel mixture.
This ignition, whose triggering time is determined by a calculator, referred to as engine calculator, usually part of an engine, is thus dependent on the arc quality, that is mainly on arc duration and on the electric parameters, such as the current intensity, the voltage. The ignition coils which are currently used in engines have been subjected to many tests to determine empirically their parameters so as to act as efficiently as possible on the spark plugs regarding the voltage applied to the secondary and the duration and current of the arc.
The primary voltage is therefore varied in order to simultaneously vary the secondary voltage until the ideal working point is obtained for the coil.
The drawbacks thus entailed are not insignificant insofar as the maximum capacities of the coils are readily reached because the primary of these coils is rapidly saturated with primary current.
Furthermore, in difficult contexts (lean mixture, high rate of recirculated gas, . . . ) leading to a poor ignition quality of the air/fuel mixture or even to the absence of ignition, it is impossible to determine which parameter (voltage, duration or intensity of the arc, . . . ) could be concerned, which consequently leads to use coils with maximum capacities in all the domains, thus increasing its cost price.
An article entitled “Capacitor Discharge Ignition System”, published in July 1992 under reference No. RD 33975, describes an ignition device comprising an ignition coil whose primary is supplied by a voltage-carrying capacitor and a secondary connected to a spark plug.
The primary of this coil is furthermore connected in series to a field-effect transistor acting as an interruptor whose opening and closing phases are controlled by a command.
Thus, when the command actuates the field effect transistor (switch) in a closed position, the capacitor is discharged in the primary, creating a voltage in the secondary in order to supply under high voltage the spark plug and to generate the arc or the spark at the electrodes thereof.
After a short primary supply time, the command actuates the switch in open position for a short time, which interrupts supply of the primary and leads to a progressive current drop thereof. Then this switch is closed before the current in the primary and in the secondary is zero. These switch opening and closing commands are carried out repetitively to obtain an electric arc whose intensity decreases with time.
This layout allows configuring the parameters of this ignition device (duration of the electric arc at the spark plug electrodes, voltage applicable to the secondary) only for a single type of use.
If it is desired to modify these parameters for another type of coil, it is necessary to establish another type of command for closing and opening of the switch, or even to modify the primary and the secondary of the new coil.
Furthermore, as in the case of conventional ignition coils, it is impossible to monitor the working parameters of this ignition, such as the secondary voltage or the duration of the arc or its intensity, to be able to correct them in a case of a deviation in relation to reference values.